Abstract:Fiber Bragg grating (FBG) is the most widely used optical fiber sensor due to its compact size, high sensitivity, and easiness for multiplexing. Conventional FBGs fabricated by using an ultraviolet (UV) laser phase-mask method require the sensitization of the optical fiber and could not be used at high temperatures. Recently, the fabrication of FBGs by using a femtosecond laser has attracted extensive interests due to its excellent flexibility in creating FBGs array or special FBGs with complex spectra. The fe… Show more
“…Since the first FBG was fabricated on a silica optical fiber by Hill et al in 1978 [ 2 ], UV lasers and phase masks have been the most popular devices for FBG manufacturing. However, driven by the demands for novel FBGs able to operate in high temperature environments (400 °C to 1800 °C) or used as vector bending sensors, femtosecond lasers are widely becoming extremely useful because of ultra-short pulse width and extreme-high peak power, which can induce fiber core refractive index change in diverse transparent materials, such as silica, single crystal, glasses, etc., [ 3 ]. Compared to phase mask technology and holographic interferometry, direct writing technologies, such as point-by-point (PbP) and line-by-line (LbL) for FBG inscriptions, demonstrate unique advantages of high accuracy with reduced thermal effect, since refractive indices could be modified to the size of sub-microns inside transparent materials by focusing a femtosecond laser beam by a high NA objective with high magnification [ 4 , 5 , 6 , 7 ].…”
We experimentally report fiber Bragg gratings (FBGs) in a single mode step-index polymer optical fiber (POF) with a core made of TOPAS and cladding made of ZEONEX using 520 nm femtosecond pulses and a point-by-point (PbP) inscription method. With different pulse energies between 69.8 nJ and 80.4 nJ, 12 FBGs are distributed along the cores of two pieces of POFs with negative averaged effective index change up to ~6 × 10−4 in the TOPAS. For POF 1 with FBGs 1–6, the highest reflectivity 45.1% is obtained with a pulse energy of 76.1 nJ. After inscription, good grating stability is reported. Thanks to the post-annealing at 125 °C for 24 h, after cooling the grating reflectivity increases by ~10%. For POF 2 with FBGs 7–12, similar FBG data are obtained showing good reproducibility. Then, the FBGs are annealed at 125 °C for 78 h, and the average reflectivity of the FBGs during the annealing process increases by ~50% compared to that before the annealing, which could be potentially applied to humidity insensitive high temperature measurement.
“…Since the first FBG was fabricated on a silica optical fiber by Hill et al in 1978 [ 2 ], UV lasers and phase masks have been the most popular devices for FBG manufacturing. However, driven by the demands for novel FBGs able to operate in high temperature environments (400 °C to 1800 °C) or used as vector bending sensors, femtosecond lasers are widely becoming extremely useful because of ultra-short pulse width and extreme-high peak power, which can induce fiber core refractive index change in diverse transparent materials, such as silica, single crystal, glasses, etc., [ 3 ]. Compared to phase mask technology and holographic interferometry, direct writing technologies, such as point-by-point (PbP) and line-by-line (LbL) for FBG inscriptions, demonstrate unique advantages of high accuracy with reduced thermal effect, since refractive indices could be modified to the size of sub-microns inside transparent materials by focusing a femtosecond laser beam by a high NA objective with high magnification [ 4 , 5 , 6 , 7 ].…”
We experimentally report fiber Bragg gratings (FBGs) in a single mode step-index polymer optical fiber (POF) with a core made of TOPAS and cladding made of ZEONEX using 520 nm femtosecond pulses and a point-by-point (PbP) inscription method. With different pulse energies between 69.8 nJ and 80.4 nJ, 12 FBGs are distributed along the cores of two pieces of POFs with negative averaged effective index change up to ~6 × 10−4 in the TOPAS. For POF 1 with FBGs 1–6, the highest reflectivity 45.1% is obtained with a pulse energy of 76.1 nJ. After inscription, good grating stability is reported. Thanks to the post-annealing at 125 °C for 24 h, after cooling the grating reflectivity increases by ~10%. For POF 2 with FBGs 7–12, similar FBG data are obtained showing good reproducibility. Then, the FBGs are annealed at 125 °C for 78 h, and the average reflectivity of the FBGs during the annealing process increases by ~50% compared to that before the annealing, which could be potentially applied to humidity insensitive high temperature measurement.
“…The manufacturing of two commercial probes from FemtoFiberTec with 6-FsFBGs sensors each was considered (instead of only one probe with 12-FsFBGs sensors), since this choice will ease the future instrumentation process of the central receiver prototype and will make the sensor configuration more robust against possible breaks and failures. In the two probes, the FsFBGs were inscribed in the single mode fiber using the femtosecond point by point FBG manufacturing process using a laser beam from an 800 nm Titanium Sapphire laser in order to focus tightly into the fiber core [ 31 , 32 ]. FBGs inscribed by infrared femtosecond lasers are thermally stable at temperatures up to 900 °C [ 33 ].…”
This work deals with the application of femtosecond-laser-inscribed fiber Bragg gratings (FsFBGs) for monitoring the internal high-temperature surface distribution (HTSD) in solar receivers of concentrating solar power (CSP) plants. The fiber-optic sensor system is composed of 12 FsFBGs measuring points distributed on an area of 0.4 m2, which leads to obtain the temperature map at the receiver by means of two-dimensional interpolation. An analysis of the FsFBG performance in harsh environment was also conducted. It describes the influence of calibration functions in high-temperature measurements, determines a required 10 nm spectral interval for measuring temperatures in the range from 0 to 700 °C, and reveals wavelength peak tolerances in the FsFBG fabrication process. Results demonstrate the viability and reliability of this measuring technique, with temperature measurements up to 566 °C.
“…All three sensors have a commonality, which is that they have a comb-like, periodic structural variation in the refraction index within the fibre core that induces a coupling action between the core mode and other modes supported by the fibre. There are a number of different fabrication methods in use; for example, ultraviolet (UV) phase-mask inscription, UV point to point, direct-write femto-second laser inscription, and fusion-arc [52][53][54][55]. The mode coupling mechanism depends on the type of grating, physical geometry, and the material used in the fabrication process, and the core mode can be guided, lossy/leaky, and radiative [56,57].…”
At the present time, there are major concerns regarding global warming and the possible catastrophic influence of greenhouse gases on climate change has spurred the research community to investigate and develop new gas-sensing methods and devices for remote and continuous sensing. Furthermore, there are a myriad of workplaces, such as petrochemical and pharmacological industries, where reliable remote gas tests are needed so that operatives have a safe working environment. The authors have concentrated their efforts on optical fibre sensing of gases, as we became aware of their increasing range of applications. Optical fibre gas sensors are capable of remote sensing, working in various environments, and have the potential to outperform conventional metal oxide semiconductor (MOS) gas sensors. Researchers are studying a number of configurations and mechanisms to detect specific gases and ways to enhance their performances. Evidence is growing that optical fibre gas sensors are superior in a number of ways, and are likely to replace MOS gas sensors in some application areas. All sensors use a transducer to produce chemical selectivity by means of an overlay coating material that yields a binding reaction. A number of different structural designs have been, and are, under investigation. Examples include tilted Bragg gratings and long period gratings embedded in optical fibres, as well as surface plasmon resonance and intra-cavity absorption. The authors believe that a review of optical fibre gas sensing is now timely and appropriate, as it will assist current researchers and encourage research into new photonic methods and techniques.
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